It used to be common practice to cache useful routines (e.g. start with "os_path_join = os.path.join" before the loop and call "os_path_join" instead of "os.path.join"), thus avoiding the iterative lookup on each iteration.

I'll admit to being skeptical to how big an effect this would have, but it was bigger than I thought:

  %%timeit
  s=0
  for i in itertools.repeat(1.0, 1000000):
      s+=math.sin(i)

  10 loops, best of 3: 124 ms per loop

vs

  %%timeit
  s=0
  math_sin=math.sin
  for i in itertools.repeat(1.0, 1000000):
      s+=math_sin(i)

  10 loops, best of 3: 89.1 ms per loop